In general, a lithographic printing plate is composed of an oleophilic image area accepting ink and a hydrophilic non-image area accepting dampening water in the process of printing. Lithographic printing is a printing method utilizing the nature of water and oily ink to repel with each other and comprising rendering the oleophilic image area of the lithographic printing plate to an ink-receptive area and the hydrophilic non-image area thereof to a dampening water-receptive area (ink-unreceptive area), thereby making a difference in adherence of the ink on the surface of the lithographic printing plate, depositing the ink only to the image area, and then transferring the ink to a printing material, for example, paper.
In order to produce the lithographic printing plate, a lithographic printing plate precursor (PS plate) comprising a hydrophilic support having provided thereon an oleophilic photosensitive resin layer (image-forming layer) has heretofore been broadly used. Ordinarily, the lithographic printing plate is obtained by conducting plate making according to a method of exposing the lithographic printing plate precursor through an original, for example, a lith film, and then while leaving the image-forming layer corresponding to the image area, removing the unnecessary image-forming layer corresponding to the non-image area by dissolving with an alkaline developer or a developer containing an organic solvent thereby revealing the hydrophilic surface of support.
In the hitherto known plate making process of lithographic printing plate precursor, after exposure, the step of removing the unnecessary image-forming layer by dissolving, for example, with a developer is required. However, it is one of the subjects to save or simplify such an additional wet treatment described above. Particularly, since disposal of liquid wastes discharged accompanying the wet treatment has become a great concern throughout the field of industry in view of the consideration for global environment in recent years, the demand for the solution of the above-described subject has been increased more and more.
As one of simple plate making methods in response to the above-described requirement, a method referred to as on-press development has been proposed wherein a lithographic printing plate precursor having an image-forming layer capable of being removed in its unnecessary areas during a conventional printing process is used and after exposure, the unnecessary area of the image-forming layer is removed on a printing machine to prepare a lithographic printing plate.
Specific methods of the on-press development include, for example, a method of using a lithographic printing plate precursor having an image-forming layer that can be dissolved or dispersed in dampening water, an ink solvent or an emulsion of dampening water and ink, a method of mechanically removing an image-forming layer by contact with rollers or a blanket cylinder of a printing machine, and a method of lowering cohesion of an image-forming layer or adhesion between an image-forming layer and a support upon penetration of dampening water, ink solvent or the like and then mechanically removing the image-forming layer by contact with rollers or a blanket cylinder of a printing machine.
In the specification, unless otherwise indicated particularly, the term “development processing step” means a step of using an apparatus (ordinarily, an automatic developing machine) other than a printing machine and removing an unexposed area in an image-forming layer of a lithographic printing plate precursor upon contact with liquid (ordinarily, an alkaline developer) thereby revealing a hydrophilic surface of support. The term “on-press development” means a method or a step of removing an unexposed area in an image-forming layer of a lithographic printing plate precursor upon contact with liquid (ordinarily, printing ink and/or dampening water) by using a printing machine thereby revealing a hydrophilic surface of support.
On the other hand, digitalized technique of electronically processing, accumulating and outputting image information using a computer has been popularized in recent years, and various new image-outputting systems responding to the digitalized technique have been put into practical use. Correspondingly, attention has been drawn to a computer-to-plate technique of carrying digitalized image information on highly converging radiation, for example, a laser beam and conducting scanning exposure of a lithographic printing plate precursor with the radiation thereby directly preparing a lithographic printing plate without using a lith film. Thus, it is one of the important technical subjects to obtain a lithographic printing plate precursor adaptable to the technique described above.
In the simplification of plate making operation as described above, a system using an image-forming layer capable of being handled in a bright room or under a yellow lump and a light source is preferable from the standpoint of workability.
As such a laser light source, a semiconductor laser emitting an infrared ray having a wavelength of 760 to 1,200 and a solid laser, for example, YAG laser, are extremely useful because these lasers having a large output and a small size are inexpensively available. An UV laser can also be used.
As the lithographic printing plate precursor of on-press development type capable of conducting image-forming with an infrared laser, for example, a lithographic printing plate precursor having provided on a hydrophilic support, an image-forming layer in which hydrophobic thermoplastic polymer particles are dispersed in a hydrophilic binder is described in Japanese Patent 2938397. It is described in Japanese Patent 2938397 that the lithographic printing plate precursor is exposed to an infrared laser to agglomerate the hydrophobic thermoplastic polymer particles by heat thereby forming an image and mounted on a plate cylinder of a printing machine to be able to carry out on-press development by supplying dampening water and/or ink.
Further, a lithographic printing plate precursor having provided on a hydrophilic support, microcapsules containing a polymerizable compound (polymerizable monomer) encapsulated therein is described in JP-A-2001-277740 (the term “JP-A” as used herein means an “unexamined published Japanese patent application”) and JP-A-2001-277742.
Moreover, a lithographic printing plate precursor having provided on a support, a photosensitive layer (an image-forming layer) containing an infrared absorbing agent, a radical polymerization initiator and a polymerizable compound is described in JP-A-2002-287334.
In general, an operation for inspection and discrimination of image formed on a printing plate is carried out in order to examine whether the image is recorded on the printing plate as intended, in advance of mounting the printing plate on a printing machine. In a conventional lithographic printing plate precursor subjected to the development processing step, it is ordinarily easily performed to confirm the image formed (inspect the plate) after the plate making (after the development processing) and before the printing (before mounting the printing plate on a printing machine) by means of coloration of the image-forming layer.
However, with respect to the lithographic printing plate precursor of the on-press development type without conducting the development processing prior to printing, the image is not recognized on the printing plate in the step of mounting it on a printing machine, and thus the discrimination of the printing plate can not be performed. In particular, it is important in the printing operation to determine whether a registry guide (register mark) which acts as a landmark for the registering in multicolor printing is recorded. Therefore, in the lithographic printing plate precursor of the on-press development type, a means for confirming the image, that is, color formation or decoloration in the exposed or heated area, is required.
The image obtained by discoloration of the exposed or heated area is referred to as a print-out image and various methods are proposed as means for obtaining the print-out image. However, means satisfying all of the plate inspection property, on-press development property and printing aptitude has not been found. For instance, a printing plate has been proposed wherein a compound capable of generating an acid, base or radical by means of light or heat and a compound capable of undergoing color change upon interaction with the acid, base or radical generated are used as a print-out agent (for example, see JP-A-11-277927). Also, it has been proposed to utilize color change of thermally decomposable compound as the print-out agent of a direct-thawing type lithographic printing plate precursor having a heat-sensitive layer (for example, see JP-A-2000-335129). Further, it has been proposed to use a thermally decomposable dye having a thermally decomposable temperature of 250° C. or below as the print-out agent (for example, see JP-A-2003-191657).
According to these proposals, the color formation occurs in the exposed area and the image confirmation property increases to some extent. However, it is still insufficient for conducting the operation for plate inspection.
It is also proposed that a specific dye precursor is incorporated into a lithographic printing plate precursor capable of image drawing by heat-mode exposure and directly mounting on a printing machine without conducting development processing to perform printing (see JP-A-2005-96219). However, even in the technique although the coloration in the exposed area is increased to some extent, it is still insufficient for conducting the operation for plate inspection and has a problem in that the plate inspection property (color-forming property) degrades with the lapse of time during preservation of the lithographic printing plate precursor. Moreover, the on-press development property and printing aptitude, for example, ink-receptive property or printing durability are insufficient.